The present invention relates to power source apparatus that AC drive a cold cathode tube, and more particularly to a field of techniques for adjusting an alternate current drive waveform to be applied to a cold cathode tube for use in a display device.
In recent electronic apparatus, liquid crystal display devices excellent in miniaturization and space saving as display devices from which users acquire information are greatly used instead of the conventional Brown tubes.
FIG. 2 shows an example of one of the conventional liquid crystal display devices with a backlight that are used generally in information processing apparatus. The backlight 5 is a light source. A liquid crystal panel 6 adjusts the transitivity of each of the display pixels of the display device using a liquid crystal control circuit 7 and hence a quantity of light coming from the backlight 7, thereby displaying a picture. Some of the liquid crystal display devices put to practical use at present can differ in backlight position and/or drive system in which the display pixels of the liquid crystal panel are driven. With transmissive liquid crystal display devices, a similar structure is employed in which a picture is displayed using light emitted by the backlight.
Although EL elements and LEDs are used as the backlights, cold cathode tubes are generally diffused. Since the cold cathode tubes have high luminosity efficiency and can be produced at low cost, they are very excellent as light sources for transmissive liquid crystal display devices. FIG. 3 shows the internal structure of the cold cathode tube, which encloses mercury 10 and an inert gas such as neon or argon with the internal surface of the tube coated with a fluorescent paint. When an alternate current voltage of several hundred volts is applied across a pair of electrodes 8 each provided at a respective one of both ends of the tube, the mercury enclosed within the tube is excited, thereby emitting UV rays. When the UV rays are applied to the fluorescent paint 9 coated on the inner surface of the tube, the fluorescent paint becomes luminescent, thereby performing a role as a light source.
When this cathode tube is driven repeatedly, the internal mercury changes to amalgam or ineffective mercury, which is any longer excited and cannot contribute to luminescence. Thus, the life of the cathode tube is determined depending on the quantity of mercury enclosed within the tube, but the enclosed quantity of mercury is limited by the size of the tube. Further, the life of the cathode tube is influenced by a quantity of current driven into the tube and the environmental temperature of the tube. Thus, how to prolong the life of the tube has been hitherto studied.
The life modes of the cathode include the simple consumption of mercury as well as an uneven longitudinal distribution of mercury within the tube. FIG. 4 schematically illustrates a life mode in this case. Usually, mercury is uniformly distributed longitudinally within the tube such that the whole tube gets luminescent. When the tube is placed in a state called cataphoresis in which the mercury within the tube moves to one end side of the tube for some reason, the other end side of the tube where no mercury is present cannot become luminescent although the whole quantity of mercury within the tube is not consumed. Thus, the one end side of the display becomes extremely dark, as viewed from a user, and the display device cannot be used, which also is a kind of life mode.
For example, JPA-2005-025981 discloses a technique that proposes a technique for preventing uneven longitudinal luminosity in the cold cathode tube.